Bar code scanners are used widely in many diverse applications. Bar code scanners offer a fairly simple means of monitoring large volumes of items. Bar code scanners are used, for example, to track inventory, at check-out areas of retail stores, and in assembly line manufacturing applications, for examples.
In bar code technology, a bar code label typically includes a series of parallel dark and light bars of different widths that represent a unique code. Bar code scanners are used to optically detect the code represented by the bar code label. A bar code scanner typically includes a laser light source, a photodetector, an amplifier, a filter and processing circuitry. The laser light source provides a laser beam which is swept across, and reflects from, the bar code label. The light reflected from the bar code label scatters, and some of the scattered light is received by the photodetector. The photodetector produces an analog electric signal. The amplitude of the analog signal represents the width of one of the series of light and dark bars of the bar code label. The amplifier receives the analog electric signal produced by the photodetector and amplifies the signal.
The amplified electric signal then is provided to a filter, typically a bandpass analog filter, which reduces signal components of the amplified electric signal outside of the frequency band of interest. The filtered electric signal then is provided to the processing circuitry (which can include a combination of analog and digital circuitry) for processing the signal to determine the code represented by the bar code label.
With such processing circuitry, it is desirable to achieve a high signal-to-noise ratio to ensure that the code represented by the bar code label is accurately detected. The signal-to-noise ratio of the circuitry depends on many factors including, for example, the precise quality of the bar code label, the accuracy with which the laser light beam is swept across the label, the background illuminance of the bar code label, the quality of the circuitry, the intensity of the input laser beam and the ambient light noise present.
Bar code scanners sometimes are used in environments in which ambient modulated light (e.g., fluorescent light) is present. In such environments, the ambient light modulation is detected by the photodetector and processed by the processing circuitry thereby reducing the signal-to-noise ratio of the circuitry and potentially causing a faulty reading of the bar code label.
As such, it is a general object of the present invention to provide simple, yet accurate bar code scanner circuitry for reducing the negative effects of modulated ambient light.